SST dies include light-emitting diodes (“LEDs”), organic light emitting diodes (“OLEDs”), polymer light-emitting diodes (“PLEDS”), and other types of light emitting dies. The energy efficiency and small size of SST dies has led to the proliferation of these devices in a multitude of products. For example, televisions, computer monitors, mobile phones, digital cameras, and other electronic devices utilize LEDs for image generation, object illumination (e.g., camera flashes) and/or backlighting. LEDs are also used for signage, indoor and outdoor lighting, traffic lights, and other types of illumination. Improved fabrication techniques for these semiconductor devices have both lowered device cost and increased device efficiency.
FIGS. 1A-1C illustrate a process for forming an SST die where the growth substrate is completely removed and a separate support substrate is attached to support the semiconductor materials. FIG. 1A illustrates an SST die 10 formed by growing epitaxial layers, including an N-type gallium nitride (“GaN”) material 12, an active region 14, and a P-type GaN material 16, on a growth substrate 20 to form an SST structure 22. The active region 14 can be a light-emitting indium gallium nitride (“InGaN”) material sandwiched between the N-type and P-type semiconductor materials 12 and 16. The growth substrate 20 is typically either sapphire, silicon carbide (“SiC”), silicon, or SiC-on-insulator (SiCOI). The growth substrate 20 can alternatively be an engineered substrate, such as silicon on poly-aluminum nitride.
It is sometimes desirable to remove the growth substrate 20 to improve the optical properties of the SST die 10 or to gain electrical access to the SST structure 22. For example, growth substrates, in particular engineered substrates, are typically opaque and thus will block emission of light produced by the SST structure 22 if the growth substrate 20 is not removed. However, since the epitaxial layers 12, 14, and 16 are extremely delicate and thin (e.g., less than 10 microns), the outer epitaxial layer 16 of the SST die 10 must first be attached to a support substrate 24 before removing the growth substrate 20. As shown in FIG. 1B, the SST structure 22 is sandwiched between the growth substrate 20 and the support substrate 24. FIG. 1C shows the SST die 10 after the growth substrate 20 has been removed in its entirety by known processes. In production, a wafer having a large number of SST dies 10 is processed to form the SST structure 22, and the support substrate 24 has the same form factor as the wafer. After the growth substrate 20 is removed, the assembly is then diced to singulate the individual SST dies 10 for mounting in a package.
One drawback of the method shown in FIGS. 1A-1C is that the growth substrate 20 is completely sacrificed, which adds both time and material costs. Another drawback is that the support substrate itself is often fairly thick and adds to the thickness of the resulting device. Furthermore, the process of bonding the support substrate 24 to the SST structure 22 is costly and may damage the support structure 22. As such, the method described with respect to FIGS. 1A-1C is capital intensive and expensive to perform.
Many SSL designs address this issue by using optically transmissive substrates, such as sapphire. As a result, sapphire substrates are not removed from the front side of the die. However, sapphire is expensive and thin (e.g., 0.15 to 5 microns) and requires a thicker conductive plate on the back side of the device. A thicker conductive plate can induce stress in the die and increases cost of production. Accordingly, several improvements in support structures of SST dies may be desirable.